The present paper intends to investigate the lexical gap-filling behaviour of primary and secondary Spanish learners of English. When there is a mismatch between the learners lexical knowledge and their communicative needs, then a lexical gap arises. Learners resort to different mechanisms to compensate for that lack of lexical knowledge. Basically, we distinguish among several L1- and L2-based mechanisms. We analysed the lexical gap-filling behaviour of 203 learners in their written compositions. We collected data when students were in 4th grade of Primary Education and four years later when they were in 8th grade. Results revealed that as learners progress in their learning of the foreign language they gradually abandon use of the L1 and substitute L1-based mechanisms such as borrowings, coinages and calques by other L2-based ones such as semantic and formal approximations, circumlocutions, and use of general words. We observed that L2 proficiency interrelates with L1 and L2 influence in different ways depending on the specific mechanism used, so that some mechanisms which are frequent at 4th grade decrease their presence, basically borrowing, and some others tend to increase, basically L2-based mechanisms as linguistic proficiency, awareness and cognitive development increase. (Contains 7 tables.)